When considering security levels related to Internet-of-things systems, some organisations may consider the use of wireless data as a possible method of allowing security breaches, whether real or imagined. In these situations it becomes necessary to remain with hard-wired networks throughout the premises. However the power requirements of the node units may not be met easily, so alternative methods of supplying power need to be investigaed. One of the more popular methods which we explain further is Power over Ethernet.
Fundamentally, Power over Ethernet (PoE) passes electrical power along with data in an Ethernet (Category 5 or Category 6 twisted pair) cable, allowing both data and power to be provided to an Ethernet-connected device with moderately low power needs with the convenience of installing only one cable. This is particularly valuable where devices are installed in decentralised locations that would otherwise require large cable runs such as IP security cameras, wireless LAN access points, IP phones, home automation and access control systems.
Some advocates of the technology expect PoE to become a widely adopted global standard for low power DC distribution to networking equipment and consumer electronics in the longer term, replacing plugpack power supplies with a centralised system that is more energy efficient, standardised and interoperable, better protected against transients and can be centrally managed and controlled by network infrastructure.
PoE can be implemented in different ways, but the basic principles remain the same. Power is supplied to a PoE network by a Power Sourcing Equipment (PSE) device, which can be either an endspan device – a network switch with built-in PoE support – or a midspan device, such as a PoE "injector" which is connected between an unpowered Ethernet switch switch and the powered device, injecting power without interfering with the operation of the switch.
The Ethernet-connected device that is supplied with power is termed the Powered Device or PD. PoE-enabled endspan switches are typically used in new installations or where an existing switch is being replaced. Alternatively, if existing non-PoE-enabled network switches are already installed and replacing them is not required then PoE midspan injectors can be installed after the switches, adding PoE support to the network at a relatively low cost.
Standard IEEE 802.3af/at PoE can be operated in one of two modes, A or B. Mode A delivers power on the data pairs of the four-pair twisted-pair Ethernet cable by applying a common-mode voltage to each pair. Because Ethernet is a differential signalling system, this does not interfere with data transmission and the common-mode voltage is supplied at the PSE end, and extracted at the PD end, though connections to the centre taps on the isolation pulse transformers.
The polarity of the power supply is ambiguously defined because of the possibility of crossover cables being connected within the network. Whilst the switch will take care of this where the data is concerned using auto-MDIX, the PD needs to be designed to accommodate a possible polarity reversal, generally by incorporating a bridge rectifier after the pulse transformer.
In a Mode B system, pins 4 and 5 of the 8P8C connector connect to one side of the DC supply while pins 7 and 8 connect to the other side of the supply. In a 10/100 Mbit/s Ethernet system these are unused pins, meaning that passive PoE connections can be supplied over these pairs with little or no electronics required at either end.
In the case of gigabit Ethernet, however, there are no spare pairs and a Mode A system is always used. The same connection scheme used for Mode B 802.3a PoE can be used in a simple, passive non-standard PoE implementation, without the negotiation that takes place between 802.3a standard PSE devices and powered devices. This sort of system is often used in low-cost PoE implementations in 10/100 Mbit/s networks.
In this type of system, the power injector does not communicate with the powered device, but simply supplies the device with power at all times. It is the responsibility of the person setting up the system to ensure that the power supply used in this type of "dumb" midspan injector is matched to the electrical requirements of the powered device.
The 802.3af standard provides up to 15.4 W of DC power (at least 350 mA at 44 volts) from each powered switch port. Only 12.95 W is assured to be available to each powered device, allowing a budget of 2.45 W of power dissipation along the length of the cable.
The 802.3at standard defines the power available to the powered device to be at least 25.5W (600 mA at 42.5V), with the maximum power being delivered from the PSE being 34.2 W (600 mA at 57V). The voltage range at the PSE is 50-57V, dropping down to somewhere between 57 to 42.5V at the load end, which must be an acceptable voltage to the powered device. For a maximum allowable voltage drop of 7.5 volts at 600 mA, the total resistance of both pairs must be no more than 12.5 ohms across their length.
The IEEE 802.3a standards also provide for signalling between the PSE and the powered device, allowing the presence of a standard-compliant powered device to be detected by the PSE and allowing the PD and PSE to "negotiate" the amount of power required and the amount available.
A powered device indicates that it is standards-compliant by placing a 25 kiloohm resistor across the powered pairs. If the PSE detects a resistance that is too high or too low, no power is applied, protecting devices that do not support the relatively high voltages of standard 802.3a PoE. An optional "power class" feature allows the powered device to indicate the range of its power requirements by changing this resistor to different values.
The power negotiation process follows simple rules – the powered device may not request more power than that allowed by the overall limits of the 802.3af or 802.3at standards, and the powered device must not draw more power than what is allocated to it by the PSE.
The PSE may deny power to any powered device that attempts to draw more power than it is allocated, but the PSE shall not reduce the power allocated to the powered device while that device remains in use. To stay powered, the powered device must continuously draw at least 5 mA for at least 60 milliseconds with no more than 400 ms since it was last active, or else that port will be unpowered by the PSE.
As you can now see, PoE provides an efficient and intelligent method of powering devices using existing network cable and small additions to infrastructure – and is certainly viable when wireless is out of the question and costs of adding power outlets for nodes could get out of hand.
If your organisation is facing these issues, or looking to create or modify a device to work with Power over Ethernet standards, we can work with you to finish the task, within your required time-frame and your budget.
For more information or a confidential discussion about your ideas and how we can help bring them to life – click here to contact us, or telephone 1800 810 124.
LX is an award-winning electronics design company based in Sydney, Australia. LX services include full turnkey design, electronics, hardware, software and firmware design. LX specialises in embedded systems and wireless technologies design. http://www.lx-group.com.au
Published by LX Pty Ltd for itself and the LX Group of companies, including LX Design House, LX Solutions and LX Consulting, LX Innovations.
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